This invention relates generally to an electrophotographic printing machine, and more particularly concerns an optical system for producing half tone images of an original document.
The process of electrophotographic printing comprises exposing a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the original document. A development system moves a developer mix of carrier granules and toner particles into contact with the latent image recorded in the photoconductive surface. The toner particules are attracted electrostatically to the latent image from the carrier granules forming a powder image on the latent image. Subsequently, the toner powder image is transferred to a sheet of support material. After transfer, the sheet of support material passes through a fusing device which permanently affixes the toner powder image thereto.
Essentially, a multi-color printing machine repeats the foregoing process a plurality of cycles, each cycle being for a discrete color. In this process, the light image is filtered to record an electrostatic latent image on the photoconductive surface corresponding to a single color. The single color electrostatic latent image is developed with toner particles complementary in color to the filtered light image employed in the formation thereof. The toner powder image is then transferred to the sheet of support material. The foregoing process is repeated for successively differently colored light images. In this manner, a plurality of toner powder images are transferred, in superposed registration, to the sheet of support material. The multi-layered toner powder image is then fused to the sheet of support material forming a permanent colored copy of the original document.
However, in conventional electrophotographic printing machines toner gradations are difficult to form. Frequently, screening methods are employed to overcome this defect. Such methods produce the effect of toner gradations by variations in dot size. In the highlight zones, the dots will be small and increase in size through the intermediate shades until they merge together in the shadow region. At the extremes, there will be complete whiteness at the highlight end of the tone scale, and nearly solid black at the shadow end. An example of this is found in U.S. Pat. No. 2,598,732 issued to Walkup in 1952.
Another patent exemplifying screening is U.S. Pat. No. 3,535,036 issued to Starkweather in 1970. As disclosed therein, a light image of an original document is projected onto a charged photoconductive surface recording an electrostatic latent image thereon. Subsequently thereto, a screened light image is superimposed over the latent image recorded in the photoconductive surface.
Other patents relating to the use of screens in electrophotographic printing are: U.S. Pat. No. 3,121,010 issued to Johnson et al. in 1964; U.S. Pat. No. 3,493,381 issued to Maurer in 1970; U.S. Pat. No. 3,776,633 issued to Frosch in 1973; and U.S. Pat. No. 3,809,555 issued to Marley in 1974.
It is a primary object of the present invention to improve the optical system of an electrophotographic printing machine to produce continuous tone copies.